This invention relates to treatment of liquids to remove particulate materials and, more particularly, to methods and apparatus for treating liquids using ballasted flocculation.
In order to remove contaminants such as suspended solids, turbidity coloring and dissolved materials such as metal salts from liquids such as drinking water, process water, wastewater and the like it is conventional to introduce coagulants and flocculating agents which precipitate dissolved contaminants and induce solid particles to be combined into larger particles which settle more rapidly from the liquid being treated. It is also known to introduce a granular material such as fine sand along with the coagulants and flocculating agents into the liquid to be treated to facilitate the settling of the floc particles.
U.S. Pat. No. 4,927,543 discloses a method and installation for treating liquid by sedimentation using a granular material such as fine sand in which flocculating agents and sand are mixed with a liquid to be treated and supplied to an aggregation area where turbulence is induced to maintain insoluble flocculated material and sand in suspension. The mixture containing flocculated insolubles and the sand is passed into a sedimentation area containing an array of spaced separator plates by which solid particulates are separated from the liquid being treated. A principal disadvantage of this arrangement is the requirement for a separate tank for a settling step, which increases the footprint required. Another disadvantage is the need to convey sand horizontally for significant distances between a sludge tank, a cleaning tank and a mixing tank.
Other ballasted flocculation systems are disclosed in U.S. Pat. Nos. 5,730,864, 5,770,091, 5,800,717, 5,840,195 and 6,010,631 but none of those systems overcomes the disadvantages discussed above with respect to the system of U.S. Pat. No. 4,927,543.
Accordingly, it is an object of the present invention to provide a method and apparatus for treatment of liquids with ballasted flocculation which overcomes disadvantages of the prior art.
A further object of the invention is to provide a method and apparatus for treating liquids in which liquid contaminants are removed rapidly.
Another object of the invention is to provide a method and apparatus for treatment of liquids with ballasted flocculation which provides a more compact arrangement and operates with greater efficiency than the prior art.
These and other objects of the invention are attained by providing a ballasted flocculation liquid treatment arrangement in which liquid to be treated is mixed with flocculating agents and granular material such as fine sand in a mixing chamber and passed to a flocculating chamber in which turbulence is introduced, preferably in a toroidal circulation pattern, which is sufficient to maintain the granular material in suspension. The flocs generated in the mixing chamber and the flocculation chamber can become attached to particles of granular material which act as a ballast. The granular material and associated flocs then pass into a sludge tank where they can settle to the bottom while permitting liquid containing small particulates and unballasted flocs to return to the flocculator.
At the top of the flocculating chamber, liquid is passed upwardly through a turbulence control arrangement in which flocs and particulates are separated from the liquid and returned by gravity to the flocculator and turbulence is eliminated so that quiescent liquid passes upwardly into a clarification chamber from which any remaining particulates can fall back to the flocculator through the turbulence control arrangement. If necessary, inclined plates or settling tubes may be provided in the clarification chamber to help separate any remaining particulates from the clarified liquid. In a preferred embodiment the sludge tank is next to the mixing chamber and sludge and granular material collected at the bottom of the sludge tank are transferred by a conveyor such as an airlift, for example, to a cleaning device located above the mixing chamber from which cleaned granular material falls by gravity into the mixing chamber and sludge separated from the granular material is removed through a sludge outlet.
The turbulence control device can have any arrangement by which flocs and particulates can be separated from the liquid and the turbulence generated in the flocculation chamber is converted to an essentially non-turbulent or quiescent flow of liquid passing into the clarification chamber to avoid mixing any remaining particulates in the clarified liquid and permit separated particulates to fall back by gravity to the flocculation chamber.
In one embodiment the turbulence control arrangement includes vertically spaced layers of closely spaced bars, preferably oriented at an angle to the bars in adjacent layers, and in another embodiment it includes vertically spaced horizontal arrays of oppositely inclined baffles. A further embodiment includes a honeycomb structure with vertically oriented cells covered by a top plate having orifices leading to the clarification chamber. In another embodiment the turbulence control arrangement has an array of inclined baffles covered by a top plate formed with orifices and surrounded by a trough. In still another embodiment the turbulence control arrangement includes a porous membrane separating the flocculation chamber from the clarification chamber and in a further embodiment the turbulence control arrangement includes a series of closely spaced horizontal gratings. The turbulence control arrangement in an additional embodiment includes a shelf projecting inwardly from the walls of the chamber and inclined downwardly at an angle of about 35xc2x0 to 55xc2x0, preferably about 45xc2x0, and extending inwardly to about 10% to 20% of the width of the chamber.